Collagen is the most abundant protein found in vertebrates. Approximately 25 percent of all animal protein is collagen. Collagen is unusual among proteins in that the amino acid glycine constitutes one-third of the total amino acid content and occurs at nearly every third amino acid residue. Also, many proline amino acids are found in collagen. Collagen contains two amino acids present in very few other proteins, i.e., hydroxyproline and hydroxylysine. The sequence glycine-proline-hydroxyproline recurs frequently.
Because glycine is a very small amino acid, chains of collagen can wind tightly around one another to form a triple helix. The side chains of proline form cross-links that lock the three strands together. Additionally, mature collagen frequently contains carbohydrate units covalently attached to its hydroxylysine residues. A disaccharide of glucose and galactose is commonly found attached to strands of collagen. Still other forms of collagen can form planar sheets, which are rich in carbohydrates. For a general description of the structure of collagen, see Stryer, Biochemistry (3d ed.), W. H. Freeman & Co., San Francisco (1988); Ramachandran et al. (eds.), Biochemistry of Collagen, Plenum Press, New York (1976); Mayne et al., (eds.), Structure and Function of Collagen Types, Academic Press, New York (1987).
Collagen functions as a structural protein of tissues. It is the major fibrous element in skin, cartilage, bone, tendon, teeth, and blood vessels. Collagen is present to some extent in nearly all organs and serves to hold cells together in discrete units. It forms insoluble fibers that have a high tensile strength. Furthermore, the basic structure of collagen is modified to meet the specialized needs of particular tissues, and these are reflected in the various types of collagen that have been identified.
The several types of collagen are a family of genetically related proteins that exhibit fundamentally similar secondary and tertiary protein structures, see, Mayne et al. (eds.), Structure and Function of Collagen Types, Academic Press, New York (1987). As used herein unless otherwise specified, "collagen" refers to any of the known types of collagens.
Type I collagen, the most prevalent type and the species found in skin, tendon, bone, and cornea, is comprised of two chains of one kind, termed .alpha.1(I), and one of another, termed .alpha.2(I). Other types of collagen have three identical chains. Each of the three strands consists of about 1,000 amino acid residues, and each has a helical conformation. The three strands wind around each other to form a superhelical cable and are hydrogen bonded to each other. As mentioned above, this structure is possible because of the presence and regularity of glycine units.
Studies have shown that the presence and concentration of imino residues, proline and hydroxy-proline, are essential for generating and stabilizing the triple helical conformation of collagen, see Bhatnagar et al., pages 479-52 in Ramachandran et al. (eds.), Biochemistry of Collagen, Plenum Press, New York (1976); Bhatnagar et al., pages 429-38 in Agris (ed.), Biomolecular Structure and Function, Academic Press, New York (1978).
In short, the stability of the helical form of a single strand of collagen depends on the locking effect of proline and hydroxy-proline residues. The triple helix is further stabilized by transverse hydrogen bonding and van der Waals interactions between residues on different strands. The superhelix is sterically allowed because glycine occupies every third position in the amino acid sequence.
In addition to being a major determinant of the architecture and tensile strength of tissues, collagen participates in numerous physiologically important interactions. These include, but are not limited to, the formation of complexes with other macro-molecules such as fibronectin, the modulation of cell proliferation, the mediation of cell migration and differentiation, and the modulation of specific gene expression.
In order for such interactions to occur, the molecules on the surface of collagen fibers must exhibit molecular perspectives that are specific for recognition. This requires local conformational changes. It has been suggested that the binding of certain cells, such as platelets, may involve a conformationally perturbed region of the .alpha.1 chain of collagen, which is located approximately one-quarter of the length of the chain from the C-terminus, see Dessau et al., Biochem. J., 169:55-59 (1978); Kleinman et al., J. Biol. Chem., 253:5642-46 (1978). This region also includes the only site known to be susceptible to proteolytic cleavage by the vertebrate enzyme, collagenase, see Gross, pages 275-317 in Ramachandran et al. (eds.), Biochemistry of Collagen, Plenum Press, New York (1976); Miller et al., Biochem., 15:787-92 (1976). Additionally, this region is known to be involved in the binding of fibronectin to collagen, see Dessau et al., Biochem. J., 169:55-59 (1978); Kleinman et al., Biochem. & Biophys. Res. Comm., 72:426-32 (1976); Kleinman et al., Analyt. Biochem., 94:308-12 (1979); and is also the site of intermolecular interactions leading to fibril formation, see Silver, J. Biol. Chem., 256:4973-77 (1981).
Previous studies have shown that the three amino acid sequence, Arg-Gly-Asp, found in a variety of proteins, including collagen, may play a major role in the binding of cells, see Dedhar et al., J. Cell Biol., 104:585-93 (1987); Pierschbacher et al., J. Cell Biochem., 28:115-26 (1985). This sequence appears twice within the .alpha.1(I) chain, and one of those occurrences is within the conformationally perturbed region described above, see Kleinman et al., J. Biol. Chem., 253:5642-46 (1978).
Collagen fragments and synthetic peptide sequences corresponding to portions of collagen have been prepared and studied. Nagai et al. prepared eleven synthetic peptides by solution procedures to study substrate specificity of purified tadpole collagenase, with the synthesized peptides having the same or closely similar sequences to that occurring around the Gly-Ile bond in the position 772-773 of the .alpha..sub.1 chain. The authors proposed an eight amino acid peptide (with acetyl at the N-terminus and esterified at the C-terminus) as the best substrate for vertebrate collagenase. Nagai et al., Biochimica et Biophysica Acta, 445, 521-524 (1976).
Collagen has been suggested in mixtures or combinations with bone minerals, such as is discussed in U.S. Pat. No. 4,992,226, inventors Piez et al., issued Feb. 12, 1991. Collagen has also been suggested in combination with hydrogels for cornea implants, as illustrated by U.S. Pat. No. 4,994,081, inventors Civerchia et al, issued Feb. 19, 1991. Skin and nerve tissue repairs have been suggested through use of endodermal implants and artificial epidermis fashioned out of collagen and mucopolysaccharide, as illustrated by U.S. Pat. No. 4,060,081, inventors Yannas et al., issued Nov. 29, 1977.
However, the present materials and composites presently employed or suggested as tissue implants or for tissue repair have various shortcomings. Collagen itself appears to cause some adverse reactions within the body. Also, the manner in which collagen is reconstituted during preparations of the combinations with bone minerals, hydrogels, and so forth, tends to markedly alter the normal collagen biological activity and apparently masks some of the biologically active sites.